1. Field of the Invention
The invention relates to vises and, more particularly, to workpiece holders used to precisely position and reliably secure workpieces during machining operations such as milling, grinding, drilling and tapping, and which can be quickly changed from one set to another.
2. Discussion of Related Art
The jaws of high-precision workpiece holders are usually made of hardened steel and have precisely-machined gripping surfaces that are designed to exert great force on a workpiece, relative to its size. It is important that this great clamping force be applied as uniformly as possible to the workpiece, so that sidewise, lengthwise and vertical deflection of the workpiece during the machining operation is prevented.
High-precision workpiece holders may be free-standing, portable jigs that are adaptable for multiple uses. Alternatively, they may be designed to be assembled on, or otherwise integral with, the machining table of a particular machine tool.
Machining tables typically have a hardened-steel upper surface that contains a plurality of spaced, parallel channels. The cross section of these channels, conventionally called T-slots, has an inverted "T" shape. The workpieces are held by jaw assemblies mounted on the table. Moveable jaw assemblies each have a foot that is inserted into a respective T-slot. Each such assembly is then slid along a T-slot until it reaches the proper position on the table. The jaw assembly is then locked in position on the table.
Workpieces are customarily placed against a fixed jaw on the machining table. The moveable jaw is then pressed against the workpiece, to clamp it in position. The moveable jaw assembly comprises a flat, relatively heavy base that is attached to the T-shaped foot on the jaw assembly. This base houses a rotatable, helically threaded shaft. Rotation of the shaft causes displacement of the moveable jaw toward and away from the fixed jaw. The fixed jaw assembly is either locked in place on the machining table, or integral with the table.
The clamping force that each moveable jaw can provide depends upon the strength of the lock that secures the jaw assembly, the precision and ruggedness of the helical threads and grooves in the moveable jaw assembly and the tightness of the fit between the jaw and the base of the jaw assembly, particularly between the shaft and the base of the moveable jaw assembly. The effective clamping force of the workpiece holder is reduced whenever the jaw is deflected relative to the base or the base is deflected relative to the table. This decrease in effective clamping force compromises the accuracy and reliability of the workpiece holder. The degree of deflection increases with wear, which causes looseness between the threads of the shaft and the corresponding grooves in the base, and between the jaw and the base.
However, it is desirable to increase the stability of the workpieces in the jaws of the workpiece holder by using gripping surfaces that conform to the surface of particular workpieces. Unlike the gripping surfaces of a simple vise, which have planar gripping surfaces that merely lie in a plane normal to the movement of the jaw, these conforming gripping surfaces must be precisely aligned relative to the machine tool within that plane, so as to prevent mislocation of the workpiece.
To save time and avoid excessive wear on the surfaces of the base and jaw that contact the shaft, jaws are customarily changed by removing an entire jaw assembly from the jig or machine table. That jaw assembly is then replaced with an assembly having the required jaw shape. However, replacing the entire jaw assembly requires a costly inventory of heavy, bulky jaw assemblies. Also, fitting the entire assembly into the T-slot, sliding it into position, and locking it in place is a time-consuming procedure that increases the risk of lock failure through wear or through operator error.